boolcoder_test: fix read of uninitialized data

Only read bits that were actually written.

Change-Id: Id62c52b7804cbfb401e6e7388201406bc899ea5d

test/boolcoder_test.cc

@@ -111,7 +111,7 @@ TEST(AV1, TestTell) {
     }
     aom_stop_encode(&bw);
     aom_reader br;
-    aom_reader_init(&br, bw_buffer, kBufferSize, NULL, NULL);
+    aom_reader_init(&br, bw_buffer, bw.pos, NULL, NULL);
     uint32_t last_tell = aom_reader_tell(&br);
     uint32_t last_tell_frac = aom_reader_tell_frac(&br);
     double frac_diff_total = 0;